Create CONTRIBUTING.md

CONTRIBUTING.md

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+# Contributing
+
+This project uses [clang format][clang-format] to format code. You can run the formatter
+with `make format`.
+
+## Supporting a new `*fetch`
+
+Define a new function in [`src/stats.h`][stats-header] and
+[`src/stats.c`][stats-implementation] for your `*fetch` tool. For example, if the tool
+was called `examplefetch`, then you would define a new function `examplefetch()`.
+
+Then, update `get_stats()` to use that newly defined function. The stats should be in
+case-insensitive alphabetical order.
+
+Finally, update the section "Supported `*fetch` tools" in the [`README.md`][readme] to
+list the newly supported tool.
+
+### Updating the example output (optional)
+
+This shouldn't be updated too often, since everyone's environment is different and
+constant updates can result in too much file churn. But after a few new `*fetch` tools
+get support, it might be a good idea to update the example output in the
+[`README.md`][readme]. You can always just copy and paste output from your terminal, but
+here are some beginner-friendly examples to copy the output to your clipboard, so you
+just have to paste into your text editor.
+
+#### Unix
+
+```shell
+fetchfetch | xclip -selection clipboard
+```
+
+#### Windows
+
+```batch
+rem cmd.exe
+fetchfetch.exe | clip.exe
+```
+
+```powershell
+# PowerShell
+fetchfetch.exe | Set-Clipboard
+```
+
+[clang-format]: https://clang.llvm.org/docs/ClangFormat.html
+[readme]: ./README.md
+[stats-header]: ./src/stats.h
+[stats-implementation]: ./src/stats.c